Not a subject for every dinner table, but worth knowing in case you should be unlucky enough to get a blocked drain or a flood. At least the London Assembly knows which way the drains flow... The London assembly have released a draft document for sustainable urban drainage (SUDS) which contains this image of the main sewer network of London:

This plan shows a small fraction of London's invisible underground architecture, the scale it is plotted at reveals the geomorphology underlying this most urban of infrastructural developments.